1. Field of the Invention
The present invention relates to a die cushion apparatus in a press machine for press-shaping a metal sheet.
2. Description of the Prior Art
In the press machine provided with a die cushion apparatus in the prior art, since a sheet material (work) was kept stationary and an upper die collided against this work at a high speed V.sub.1 upon commencement of a shaping operation, a vibration and a noise were caused. Accordingly, it has been required to resolve at an early date the problem of the vibration and the noise which are increasing in accordance with speed-up of the press machine.
Even with respect to the die cushioning capability, the die cushion apparatus in tne prior art could not have its capability fully controlled due to the fact that it relied upon a pneumatic pressure only, and this served as a neck for shortening a try period of a metal mold and for allowing to complex a metal mold.
Although the required capability of the die cushion apparatus during the working process is necessary upon commencement, it is desired to reduced the capability at the midway of the working process, or it is desirable to employ a variable die cushion system in which the capability is increased just prior to finishment of the working. However, the die cushion apparatus in the prior art had only a constant capability, and could not meet the requirement.
The present invention has been worked out under the above-mentioned situation of the prior art.